1. Field of the Invention
The present invention relates to a PFC converter for improving power factor, which is a type of AC-DC converter that receives Alternating-Current (AC) power supply and outputs a Direct-Current (DC) voltage.
2. Description of the Related Art
In a typical switching power supply apparatus using a commercial AC power supply for input power, the commercial AC power is converted to a DC voltage by rectifying and smoothing, and then the DC voltage is subjected to a switching operation in a DC-DC converter. Accordingly, an input current becomes discontinuous and is significantly distorted as compared to a sinusoidal wave, generating harmonic currents. To prevent damage caused by such harmonic currents, harmonic current regulations classified by usage or input power are imposed on power supplies of electronic equipment in Japan, Europe, etc.
To cope with these regulations, a circuit known as a PFC (power factor correction circuit) converter is added to the power supply of electronic equipment to suppress the harmonic currents. A typical PFC converter is now described with reference to FIG. 1 of Japanese Unexamined Utility Model Registration Application Publication No. 3-70085.
An input power is supplied from a commercial AC power supply Vi through a low pass filter FIL, and is converted into a pulsating voltage by a full-rectification circuit RF1. The pulsating voltage is input to a chopper circuit in the subsequent stage. The chopper circuit includes a rectifying-and-smoothing circuit formed of an inductor L1, a switching device Q1, a diode D1 and a smoothing capacitor C1. The turning-on and turning-off operations of the switching device Q1 included in the chopper circuit are controlled in such a manner that the waveform of an input current Iir becomes similar to the waveform of an input voltage Vi, i.e., a sinusoidal wave-like shape with the same phase.
A control circuit includes an error amplifier A, a circuit B for detecting when an inductor current becomes zero, a current detector F, a voltage detector G, a multiplier H, a comparator E, a pulse generator C and a driver circuit D.
The multiplier H outputs a value produced by multiplying the output of the voltage detector G by the output of the error amplifier A, which corresponds to a voltage between two terminals of the smoothing capacitor C1. When the output value of the multiplier H exceeds the output value of the current detector F, the switching device Q1 is turned off by the driver circuit D via the pulse generator C. When the current flowing through the inductor L1 becomes zero, the detection circuit B sends a signal to turn on the switching device Q1 again. By repeating these operations, the peak value of the current flowing through the inductor L1 forms a sinusoidal waveform since the output of the voltage detector G also forms a sinusoidal waveform, thereby making the average value of the current form a sinusoidal waveform as well. Consequently, the power factor is improved because of the sinusoidal input current Iir, and the harmonic currents are suppressed to a certain level or lower. This control method is known as the critical current mode method.
In addition to the critical current mode, the continuous current mode is well known as a control mode. In the continuous current mode, the switching frequency is fixed, and the turning-on and turning-off operations are controlled so that the average value of the inductor current flowing through the inductor follows a reference sinusoidal wave.
As still another control method, Japanese Unexamined Patent Application Publication No. 7-75329 describes a method in which upper and lower limits are set with minute spacing therebetween relative to a reference sinusoidal wave, and controls the turning-on and turning-off operations of the switching device so as to occur between these limits.
However, in the continuous current mode, there is a drawback in that the loss becomes large because of switching losses generated by the turning-on and the turning-off operations. In the critical current mode, the input current is limited to one-half of the inductor current peak value. Thus, there is a drawback in that application thereof for high power usage is difficult. In the other control method in which the upper and lower limits with minute spacing therebetween are set relative to the reference sinusoidal wave, it is difficult to reduce the switching losses since the range of control is limited to the minute spacing.